Reverse objects order in group
ID property sortkey

Test script to demonstrate adding an ID prop as a sort key.  Creates two groups from scene objects "Group1", the second,  "Group2", in reverse order and adds a sort key  add_key(group)  

Saving and reopening the file and group.orderred_list returns "Group2" still in reverse order, as created.

import bpy
context = bpy.context  # for testing

def add_key(group):
    # simple integer key
    key = 0
    # add an IDProp key for each group object
    for o in group.objects:
        o[group.name] = key
        key += 8

    return None


def sorted_group(group):
    def key(obj):
        # key is groupname prop if set else 0
        return obj.get(group.name, 0)

    g = [o for o in group.objects]
    g.sort(key=key)
    return g

bpy.types.Group.orderred_group = property(sorted_group)


# test calls
scene = context.scene

obs = [o for o in scene.objects]


for name in ["Group1", "Group2"]:
    print(name)
    group = bpy.data.groups.get(name)
    if not group:
        group = bpy.data.groups.new(name)
        if name == "Group2":
            obs.reverse()
        for o in obs:
            group.objects.link(o)
        add_key(group)
    elif name == "Group2":
        sl = group.orderred_group
        # reorder by unlink link
        for o in sl:
            group.objects.unlink(o)
            group.objects.link(o)

    print(group.objects.keys())